<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>项目-剩余宽高</title>
    <style>
     *{
         margin: 0;
         padding: 0;
     }
     html,body{
         height: 100%;
     }
     body{
         display: flex;
     }
     .box1,.box3{
         width: 100px;
         background: gray;
     }
     .box2{
         background: yellow;
         flex: 1;
     }
    </style>
</head>
<body>
    
        <div class="box1"></div>
        <div class="box2"></div>
        <div class="box3"></div>
    
</body>
</html>